All-ceramic glass making system

ABSTRACT

System for producing high quality laser glass including an allceramic melter with an integral orifice tube. One important embodiment of the invention includes an all-ceramic hollow bladed stirrer with means for rotating the stirrer in any desired direction and at controllable speed.

United States Patent Silverberg 5] June 13, 1972 [54] ALL-CERAMIC GLASSMAKING 2,351,594 6/1954 Black et SYSTEM l,616,525 2/1927 Booze ..263/481,852,162 4/1932 Harris et al. ..263/48 [72] Inventor: Carl G.Silverberg, Sturbridge, Mass. 3,l74,729 3/1965 Cala ..6$/l 80 f 73]Assignee: American Optical Corporation, Primal), Examiner Jhn L Cambyswmbrldfle- Mass- Attorney-William c. Nealon, Noble s. Williams,Robert]. 22 Filed: mi. 26, 1970 Bird and Bcmard Sweeney [21] Appl. No.:14,599 57 ABSTRACT System for producing high quality laser glassincluding an all- [52] US. Cl. ..263/40 R, 65/179 ceramic melter with anintegral orifice tube. One important [51] Int. Cl ..F27b 3/00 embodimentof the invention includes an all-ceramic hollow [58] Field 0! Search..65/ l 79, 180; 263/40 R, 48 bladed stirrer with means for rotating thestirrer in any desired direction and at controllable speed. [56]References Cited UNlTED STATES PATENTS 23 Claims, 8 Drawing Figure2,64l,454 6/l953 Labino ..65/|79 X FIG.2.

PATENTED N1 I 2 3.669.435

SHEEI 10F 3 mv ENTOR CARL G. S! LVERBERG ATTORN EYS PATENTEDJun 13 m2SHEEI 2 0F 3 F|G.3. H630. FlG.3b. FIG.3c.

' INVENTOR CARL G. SILVERBERG BY 5%" ATTURE S ALL-CERAMIC GLASS MAKINGSYSTEM BACKGROUND OF THE INVENTION The production of laser glass haspresented many problems which prior to the discovery of the glass laserwere not necessary to consider when making optical quality glass. Forreasons more fully demonstrated below, laser glass in addition to beingof the highest optical quality must also be free of platinum inclusions,materials which exhibit absorption at around I am, striae, bubbles,stones and other non-metallic inclusions and must have a fluorescentlifetime of at least 500 microseconds.

Since one of the requirements for laser glass is that it be free fromstriae, platinum crucibles were extensively employed in the early stagesof laser glass development to melt the batch materials which form thelaser glass. The reason for selecting platinum as the material for thecrucible is that it was known that ceramic crucibles, the onlyreasonable alternate possibility, contributed to the formation of striaewithin the finished glass.

Striae formations in glass that is formed in ceramic systems resultsfrom the ceramic refractory dissolving into the molten glass due tochemical attack by the molten glass at the surface of the refractory. Asolution of refractory in molten glass overwhelmingly tends to result inan unhomogeneous final product with different indices of refraction(striae).

Although there have been some successful attempts at forming evenastronomical objective quality glass in allceramic melting systems, themain disadvantage of such systems is the low yield of good opticalquality glass. Due to the low yield obtainable from all-ceramic systems,the optical glass industry prior to the advent of laser glass changed toplatinum systems for economic reasons. The choice of platinum wasobvious. Platinum, a noble metal, is considered to be chemically inertin glass systems. Thus it was felt that glass formed in such platinumvessels would be free of striae. It was believed that since the platinumwould be able to withstand the chemical attack of the molten glass, thepossibility of the crucible itself becoming part of the final productwas remote. Furthermore, it was felt that, although the initial cost ofplatinum is high, since platinum is a noble metal its lifetime isindefinite. In fact, as it turns out, platinum crucibles are indeed themost acceptable way of manufacturing the highest optical quality glassfor use in such objects as lenses. However, glass that is melted inplatinum systems contains platinum inclusions. These inclusions havebeen observed with particle sizes as large as 500 pm and as small as thelimit of resolution of the optical microscope (about 2 pm). For opticalapplications the inclusion of such particles presents no problemwhatsoever. However, when a glass is to be used in laser applicationsthe inclusion of even the smallest platinum particle causes disastrousresults. The high energy which is propagated throughout a laser glasscauses platinum to vaporize, expand and crack the laser glass. Since itis not acceptable practice to remelt the laser glass, when this occursthe entire piece of cracked laser glass is completely worthless and mustbe replaced.

The occurrence of platinum inclusions in glass melted in platinumcrucibles was not completely understood by those skilled in the art.Many theories were offered as to how platinum could get into the glass.Four possible mechanisms of particle formation were advanced, namely:

1. Mechanical abrasion 2. Metallurgical change in the platinum 3.Solution of the platinum by the glass 4. Oxidation of the platinum andsubsequent reduction of the oxide Extensive investigation revealed thefourth named mechanism as the mechanism that significantly contributesto the formation of platinum in the glass.

Although platinum is thought of as an inert noble metal, it does oxidizeat elevated temperatures forming the dioxide R0,. The oxide ismetastable, decomposing to platinum metal and oxygen. Thesecharacteristics give rise to possible mechanisms for the formation ofplatinum inclusions in the laser glass, i.e., the platinum oxidizes,vaporizes and then is deposited on the surface of the glass either as anoxide, where it is subsequently reduced to the metal, or as the metalfollowing reduction of the vapor at some intermediate step.

To check the foregoing hypothesis a control experiment was conducted. Inthis experiment a sample glass was formed in a platinum crucible in airand a corresponding sample was formed in a platinum crucible over whichwas maintained an inert atmosphere of nitrogen gas. The result of theexperiment showed conclusively that under the test conditions when aninert atmosphere such as nitrogen was utilized in a platinum system theresulting glass contained fewer particles of platinum than glass formedin the same system in air.

While this experiment provided strong evidence for one mechanism bywhich platinum inclusions are found in glass, it was not a solution tothe problem. Further work indicated that platinum crucibles, whensubjected to neutral atmospheres at high temperatures (I475 I540 C.),become contaminated with small percentages of metals such as antimonyand zinc which are contained in most glasses as oxides. To compound theproblem, when the atmosphere is made reducing the platinum alloys withthese materials to their eutectic compositions. The research in thisarea indicated that the use of an inert atmosphere to solve the problemof particle formation by the transfer of platinum from the crucible tothe glass introduced another problem; namely, the formation of platinumparticles by the alloying of platinum with some of the glassingredients.

SUMMARY OF THE INVENTION In accordance with the present invention theforegoing problems are significantly solved by providing an all-ceramicmelting system of special design.

Although it might seem that the return to the all-ceramic type ofmelting would be a step backward in the optical glass technology, theresults of numerous experiments have indicated that for laser glass thisapproach is the most acceptable.

It is accordingly an object of the present invention to provide asuitable all-ceramic system for producing high optical quality laserglass.

It is a further object of the invention to provide a method forproducing a laser glass which exhibits high transmittance at the laserwavelength, high homogeneity and freedom from striae and ceramicinclusions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a somewhat schematicvertical cross-sectional view of the all-ceramic system of the presentinvention;

FIG. 2 is a perspective view of the crucible for melting glass in thesystem ofFIG. 1;

FIG. 3 is a perspective view of the stirrer of the system of FIG. 1;

FIGS. 30 3c are views of other stirrers which are used in theall-ceramic system of the present invention;

FIG. 4 is a graph showing some melting curves for melting glass in theall-ceramic melter of the present invention; and

FIG. 5 is an annealing curve for a laser glass prepared in theall-ceramic system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the presentinvention, laser glass is produced in an all-ceramic system. In thisregard it is to be undetstood that as used throughout this specificationand claims the term all-ceramic system includes components whichaccomplish fining and homogenizing, as well as melting. In connectionwith the foregoing definition of all-ceramic systems, it was found thatplatinum particles are introduced in the glass even if the batchreaction is carried out in a ceramic unit and only the fining andhomogenizing is carried out in platinum units.

As was explained above, processes of optical glass manufacture inall-ceramic systems are known. The best known of these processes is the"transfer" method. This is a pot process, using refractory ceramiccrucibles which may hold up to 2 tons of molten glass. In this process,after the glass is carefully melted and stirred very thoroughly with aceramic stirrer, it is allowed to cool slowly in the pot. During thecooling, the glass cracks in many pieces, some of which are hopefullylarge enough and of sufficient quality to be reheated and slumped intothe shape desired. It is obvious that this process is not an efficientone.

In another known process, the molten glass is poured carefully from apot to a large metal mold, with sufficient caution taken to avoidexcessive striae formation. This casting processing has been used toproduce large lens and mirror blanks for telescopes and optical windows,but usually many castings are made and rejected before one of sufficientquality is produced. The overall yield of large pieces of acceptableglass from this process, however, is superior to that resulting from thetransfer method. The main disadvantage of the foregoing process forproducing laser glass is that a glass blank of approximately 1 ton hasto be made in order to obtain one or two 30 lb. pieces of glass usablefor laser rod fabrication. Since the constituents which make up laserglass are expensive and should not be remelted, such a process iscompletely unsatisfactory. In addition, there are other problems in themelting of laser glasses in such systems. High purity refractories arenot available in large pot sizes and it is also difi'icult to pour theglass from a large pot without introducing striae.

In contrast to the foregoing known ceramic systems, the allceramicsystem of the present invention is shown in FIG. 1. At the outset thesystem is described in its most general aspects.

The batch materials which form the laser glass are melted in a crucible10 which is positioned within a furnace or oven 16. In accordance withthe invention, crucible I is constructed of a material which isresistant to chemical attack by the molten glass and which has a low ornegligible iron content. The selection of the crucible material is setforth in more detail below. As is shown in FIGS. 1 and 2, crucible isdesigned with an open top 12 and a hemispherical bottom 14. Sideportions 15 together with bottom 14 form the melter portion 13. At thegravitational center of bottom 14 is an orifice 17 which allows thepassage of molten glass into orifice tube 19 which is formed integrallyto the melter 13.

Although the construction of furnace 16 forms no part of the invention,the furnace I6 is preferably constructed of a fire-proof asbestos cementboard such as Marinite with an inner lining of a high temperaturealuminum glass fibrous board such as Ceraform board. Both Marinite andCeraform are sold by Johns Manville Corporation. The foregoingcombination (not shown in the drawing) forms the outer side of thefurnace and the inner portions of the furnace are constructed of aninsulating fire brick 2i (K30) capable of withstanding I650 C. The crown23 and bottom 25 of the furnace are formed of aluminum oxide castablerefractory.

The bottom of the furnace is made with a stepped bottom openingpermitting crucible 10 to be raised into the furnace from the bottom.

In order to melt the glass, the system is heated by radiation fromsilicon carbide resistance elements 20 which are placed on four sides ofthe furnace. Thermocouples (not shown) are provided for controlling andrecording temperatures. Separating heating elements 20 from the crucible10 is a ceramic cylindrical muffle 22. The purpose of the muffle 22 isto homogenize or even the temperature distribution in the crucible wallsand glass melt. In order to accomplish homogenization of the temperaturedistribution, muffle 22 is positioned between the crucible and theheating elements and extends vertically above and below the crucible. Inone important embodiment of the invention the muffle is formed ofsilicon carbide. However, muffles formed of other materials, forexample, refractory oxides, are usable in accordance with the presentinvention.

Homogenization of the glass (the elimination of striae) is the mostcritical problem in an all-ceramic melting system. The homogenization ofoptical glass is usually achieved by stirring. In accordance with thepresent invention, during the glass melting cycle the glass is stirredby the simultaneous operation of several stirring mechanisms.

An overhead stirrer 24 extends into crucible 10 through an opening 18 inthe top of the furnace and is driven by a stirring head (not shown)which provides the means for rotating the ceramic stirrer at variablespeeds and directions of rotation. The stirring head also permitsoscillation of the stirrer horizontally as well as vertically at speedsand amplitudes independent of the rotation.

The stirring head consists of a multiblade chuck attached to a shaftwith a ball bearing mounting. The shaft is driven by a timing belt and avariable speed gear first motor which can be operated at 2 to 40 rpm.The stirrer can also be oscillated at 0.l to 1 cycle per minute by meansof a second motor and a variable stroke cam (not shown). The first motoris mounted to a floor plate and the second motor assembly is connectedto oscillate this floor plate so that when the plate is oscillated bythe second motor the entire stirring head oscillates. The floor plateholding the stirring head is mounted on a motor driven helical screw sothat the entire stirrer can be oscillated vertically and also lifted outof or lowered into the glass melt.

The foregoing stirring head and motors are not specifically shown in afigure of the drawing since the preceding description is sufficient toenable one skilled in the art to practice this feature of the invention.

The foregoing homogenization cycles are regulated by four electroniccontrollers with feedback to compensate for load variations which arepreferably programmed through a 7-day off-on type timer.

In the preferred embodiment of the present invention, stirrer 24 isformed with blades 26 to provide a more positive displacement of themolten glass and is shown in FIGS. l and 3 with two flights of bladeswith each flight having two blades. However, improved results areobtainable with stirrers having one flight or more than two flights ofblades as is shown in FIGS. 3a and 3b.

Since contamination and striae can occur from stirrer 24 as well ascrucible 10, the choice of the material for the stirrer is as importantas the selection of the materials for the construction of crucible 10.In the preferred embodiment of the invention stirrer 24 is formed of thesame material as crucible 10.

As is shown in FIGS. 1 and 3, stirrer 24 is hollow. By providing ahollow shaft for stirrer 24 it is possible to air cool the stirrer andthereby reducing the rate of attack by the glass on the stirrerpreventing the formation of striae in the glass. In addition to theforegoing advantage, by providing the system with a hollow stirrer it isalso possible to bubble gases (air) into the molten glass throughapertures 30 strategically located at various points on the stirrerbeneath the surface of the molten glass. Thus air or oxygen can be fedinto the stirrer and bubbled into the glass which assists in stirringthe glass and preventing the formation of striae.

To further facilitate stirring, the crucible is supported on a rotatingpedestal hearth 28 which holds the glass melting crucible and which canbe controllably rotated during the glass-making period. Hearth 28 ismade of a series of refractory insulating materials and has twocylindrical heating elements 3!, 33, one above the other, builtconcentrically into the center of the hearth and which provides themeans for controlling the temperature of the orifice tube 19. Slip-ringsbuilt into the hearth provide connections for the heating elements powerand for thermocouple leads to monitor and control the temperature ofeach heating element.

The glass melting crucible rests on the rotating hearth 28 supported bya cast aluminum oxide tripod 35. Orifice tube 19 of crucible 10 extendsinto hearth 28 with its end exactly flush with the bottom of therotating hearth. Hearth 28 is supported by ball bearings with aspherical outer race and it is driven by a variable speed motor allowingspeeds from 0.1 to I rpm.

As pointed out above, the overhead stirring mechanism rotates thestirrer at variable speeds and directions, while oscillating the stirrerhorizontally and vertically at speeds and amplitudes independent of therotation. Since the crucible itself is supported on a rotating hearthwhich rotates at an independently variable speed, the combination ofmovements permits the stirrer to sweep out all parts of the melt with avariety of stirring patterns.

In the preferred embodiment, the stirrer is formed with blades toprovide a more positive displacement of the molten glass, and the shaftof the stirrer is hollow for the purpose of air cooling to reduce therate of attack by the glass and for bubbling gases; e.g., oxygen or airthrough the shaft. In the latter case, holes are provided in the blades.It is also possible to use a ceramic tube without blades but open at thelower end, as shown in FIG. 3c, if stirring is assisted by bubbling.

After the glass is thoroughly homogenized by stirring, it is cast fromthe bottom of the crucible. The hemispherical bottom of the crucible notonly favors stirring but also facilitates emptying all the glass in thecasting step. The glass is kept from running out of the chamber by aplug of glass which is cooler and more viscous than the particular melt.By heating the glass plug to its flow point, the molten laser glasspasses out through tube 19 at the bottom of the system.

In one important embodiment of the invention, the allceramic systemutilizes a thin walled mullite crucible which is heated by radiationfrom silicon carbide resistance elements 20. Numerous tests haveindicated that mullite is the preferred material for forming the ceramiccrucible component of the all-ceramic system of the present invention.It is to be understood, however, that the invention is not limited tothis particular material. Various factors control the selection of thematerial that can be effectively used to form the crucible. Since ironis known to exhibit absorption in the vicinity of 1 micron, it isimperative that the material chosen to form the crucible have a low ironcontent. The transmittance at 1.06 micron, the neodymium laser emissionwavelength, is affected most adversely by the presence of ferrous ironin the glass (the Fe iron has an absorption that peaks around 1 urn inmany glasses). For many applications, an absorption at 1.06 um of 2.5 x10" cm or less is desirable. This is almost an order of magnitude betterthan the absorption of many optical glasses. Since there are availableraw materials which permit laser glasses to be prepared that willexhibit the foregoing absorption, it is essential that the ceramicmaterial be essentially iron-free.

Refractory oxide ceramics are the most logical group of materials foruse in an allceramic melting system since they are compatible with theoxidizing conditions present in most laser glass melts. However, inaddition to a low iron content and resistance to attack by the moltenglass, the ceramics for use in forming the crucible must have goodthermal shock resistance since the semi-continuous system of the presentinvention involves cyclic thermal variations in its operation.

Numerous tests have indicated that the micro-structure of the ceramicused should be reasonably dense and fine grained for minimum formationof stones or ceramic inclusions. The foregoing micro-structure favorsgood resistance to corrosion by molten glass but is not particularlyfavorable to withstanding thermal shock resistance. Thus, a compromisewith respect to these factors has been found to be the most acceptableapproach.

01' all the materials tested, mullite proved to have reasonable thermalshock and to contribute very little iron to the glass. A high puritymullite material manufactured by The Mc- Danel Refractory PorcelainCompany is considered the best material for crucibles in accordance withthe requirements of the present invention. Such a material is given asExample XI of Table I below.

In addition to mullite, other refractory materials were tested asmaterials suitable for forming the crucible of the all-ceramic system ofthe present invention. These materials are shown in Table 1 below:

Table l Refractory Ceramic Materials Tested Constituent Composition byweight of major constituents with balance of composition consisting ofinert impurities Example I Al,0, 50. 7

Sta, 1 1.3

Modifying oxides 1.6

Example 11 A1,0, 499

SiO, 15.3

Modifying oxides 1.6

Example 111 Al,0 94.8

Na,O 3.6

Modifying oxides 1.6

Example 1V AkO, 99.4 Example V Al,0, 45.0 ZrO, 40.0 SiO, 13.5 Example VIALO, 49.0 ZrO, 34.0 SiO, 15.0 Example VII Al,0 68.4 ZrO, l8. 1 SiO, 13.2Example Vlll A1 0 70.0 ZrO, 19.5 S10, 102 Example 1X ALO 99.5 Example X210, 67.0 SiO, 33.0 Example XI (mullite) A1,0, 63.0 SiO, 37.0 ExampleXII A1,0, 960 Example XIII ALO, 99.0 Example XIV ZrO, 94.0 CaO 5.0Example XV ZrO undetermined TABLE II.RESULTS OF CERAMIC EVALUATION TESTSAbsorp- Reslstance tlon at to attack in 1 Um Fluor. Refractive indexstatic 13 (percent lifetime Example No. mm. test cm (Us) 589 nm. 1,061nm. Platinum 0.2 570 1.5194 1. 5091 reference standard (no ceramlo).

0. 5 580 l. 5205 l. 5102 1. 3 540 1. 5200 I. 5093 1. 1 570 l. 5208 l.5092 0. 4 560 1.5188 1. 5079 0.4 560 1. 5207 1. 5102 1.9 560 1.52081.5103 0.8 550 1.5223 1.5119 0. 6 1540 1. 5223 1. 5090 0.6 15401.51907 1. 5100 1.0 520 1.5225 1.5140 Not run-ceramic dissolved in meltAs is explained above, when all the various factors are evaluated,particularly resistance to attack, absorption at 1 pm and fluorescentlifetime, mullite is considered as the best material for laser glasspots or crucibles. However, from the foregoing test results, systemsmade of the materials of Examples I, IV, V], IX and X1] will enable theproduction of laser glass which is superior to laser glass made in priorart systems.

In accordance with the present invention, laser glasses were prepared inthe all-ceramic system of the present invention.

The system utilized was constructed as follows:

The furnace was 125 cm square and about cm high. It was heated on allfour sides by silicon carbide rods, 20 units in all, each having aheating section 25 mm in diameter and 60 cm in length. Thermocoupleswere provided for controlling and recording the temperatures.

The furnace was constructed using a 2.54 cm thick Marinite as theoutside portion, lined with cm of Ceraform board and l [.43 cm of [(-30brick. The crown and bottom of the furnace were cast of Norton Company'saluminum oxide castable cement, TA i034, which was also used for the topof the rotating hearth and the supports for the melting crucible.

The bottom of the furnace was made with a stepped bottom opening with aminimum opening diameter of 35 cm, permitting the crucible to be raisedinto the furnace from the bottom.

The rotating pedestal hearth was made of a series of refractoryinsulating materials. Two cylindrical heating elements, one above theother, were built concentrically into the center of the pedestal hearthwhich had a total height of 35 cm (not including crucible supports).Slip-rings built into the hearth provided connections for the heatingelements power and for thermocouple leads to monitor and control thetemperature of each heating element.

The glass melting crucible rested on the rotating hearth, supported by acast aluminum oxide tripod. The orifice tube of the crucible extendedinto he hearth with its end exactly flush with the bottom of therotating hearth. The hearth is supported by ball bearings with aspherical outer race and it is driven by a variable speed motor allowingspeeds from 0. l to l rpm.

The crucible was a pure white mullite made by the McDanel RefractoryPorcelain Company of Beaver Falls, Pennsylvania, using their ceramiccomposition which is 63% Al,,0 37% SiO by weight. The dimensions of thecrucible were 25 cm in diameter and 35 cm high with a hemisphericalbottom which blended into an orifice tube 6 cm in diameter and 40 cmlong. The walls of the crucible and orifice tube were 6 to 9 mm thick.The crucible was fully vitrified, translucent, with no visible grainstructure and all edges were chamfered.

The stirring head was placed over the top of the furnace and held apropeller-type stirrer made of the same high purity mullite. The stirrerwas [00 cm long, had a shaft 4 cm in diameter with 3 flights of 2 bladeseach, located on opposite sides of the shaft.

The 2.5 cm long blades were the same width as the shaft and werearranged perpendicular to the axis but sloped approximately 30 from thehorizontal plane. The first pair of blades was located about 7 cm fromthe lower end of the shaft. The distance between adjacent blades wasabout 7 cm.

The stirrer projected into the furnace through a 6.5 cm wide and cm longrectangular opening in the furnace top.

The stirring head consisted of a multiblade stainless steel chuckattached to a shaft with a ball bearing mounting. The shaft was drivenby a timing belt and a variable speed gear motor which could be operatedat 2 to 40 rpm. The stirrer could also be oscillated at 0.l to 1 cycleper minute by means of a second motor and a variable stroke cam.

The stirring head was mounted on a helical screw so that the entirestirrer could be lifted out or lowered into the glass melt. The verticalexcursion could also be programmed for limited up and down motion withina range from 0.5 to 10 cm per minute. The head was completely enclosedand cooled by means of a 2.83 mlmin. blower.

The furnace temperature was controlled by a 400 series Barber Colmanproportioning controller, augmented by two 290 type Barber Colmaninstruments, which provide for separate control, including relays, atpreset high and low limits. The temperatures were monitored on arecorder located at the lower floor level.

The input to the furnace could be varied by altering the voltage with atwelve-point tap switch provided on the transformer. The furnacetransformer had 6 coarse and 6 fine settings.

The homogenization cycles were regulated by 4 Minarik electroniccontrollers with feed-back to compensate for load variations which wereprogrammed through a 7-day off-on type timer.

Orifice tube temperature controllers were used to regulate the castingof the glass from the orifice tube. The input to each heater wascontrolled by means of a variable transformer and the temperature wasindicated and controlled by 290 type Barber Colman instruments. Thetemperatures were fed to a recorder for monitoring and study.

Numerous laser glasses were prepared utilizing the foregoing all-oeramicsystem.

The preparation of a representative example is given below:

Prior to loading the batch materials, the crucible was preheated to atemperature of M00 C, whereupon the crucible was withdrawn from theoven.

The amount of the batch was calculated to yield approximately 50 kg offinished glass and was prepared from raw materials using standard glassmaking weighing and mixing procedures. SiO,, ZnO, Sb,0,, and Nd,0, wereintroduced as such. U 0, Na,0, K,O, and BaO were introduced ascarbonates and/or nitrates. 441,0, was introduced as Al( 0H),.

To avoid dusting and to reduce the volatilization losses of batchconstituents during the reaction phase of the melt, all batches werepelletized. For that process, a rotating polyethylene disc 60 cm indiameter and i5 cm in depth was used. This disc rotated around an axistilted at a 45 angle relative to the vertical direction. The rate ofrotation was 30 rpm. Distilled water was continuously dropped onto thepowder. The diameter of the pellets produced was approximately 5 mm.After pelletizing, the batch was dried at l20 C for 48 hours. During theentire batch preparation process, special care was taken to avoidintroduction of organic and inorganic contaminants.

The amount of the various constituents used to prepare the glass aregiven in Table III below:

The batch shown in Table Ill was added to the preheated crucible inportions of approximately 300 g.

Melting took place at the temperatures shown in curve 40 of FIG. 4 andunder the melting conditions shown in Table IV below. The glass wasprevented from running out of the crucible by a plug of glass havingproperties as set forth above.

TABLE IV.TIME-TEMPERA'IURE-STIRRING RELATIONS Direction of Rotation:Clockwise ounterclockwise After completion of the homogenization phase,the orifice was heated to approximately 1000 C and the viscous glass wasextruded from the orifice of the melter portion of the crucible at therate of 1.5 cm/sec. through a graphite sleeve of 7.5 cm inside diameter.The outside dimensions of the sleeve were 35.5 cm in height and 25.5 cmsquare.

During this casting operation, the stirrer rotated slowly and was placedclose to the wall of the crucible in order to move striae locatedbetween the paddles of the stirrer towards the edge of the finishedbillet. The portion of the glass billet leaving the sleeve was coveredimmediately with thermally insulating material. The casting of the glasswas made into a vertical molding machine to produce a billet 8 cm indiameter and aproximately 110 cm long. After completion of the castingoperation, the covered billet was moved to the annealing furnace andannealed according to the time-temperature relation given in FIG. 5.That type of anneal resulted in birefringence of less than 10 sm/cm. Abillet of this glass was then drawn into a laser rod which had afinished dimension of cm in diameter and 200 cm in length. Thecomposition of the finished glass was:

Composition by Weight SiO, 68.52 Li,0 1.02 Na,0 7.35 K,0 1 1.13 ZnO 1.53BaO 4.90 A1,0 1.53 Sb.,0 1.02 Nd,0, 3.00

The refractive index of the rod was constant over the length and thediameter l l0"". When the rod was properly aligned in a laser cavity nodamage occurred with energy densities as high as l5.l/cm even afternumerous shots.

In addition to the foregoing example, in several melts for the samecomposition the time-temperature relation during the homogenizationphase was changed. Two of the temperature curves are indicated by dottedlines 42, 44 in FIG. 4.

Although the time-temperature relation shown by the solid curve 40yielded best results, good optical quality glass was obtained betweenall the melting conditions shown in H0. 4.

in connection with the foregoing examples, it is to be understood thatthe parameters disclosed form no part of the invention. Such details aredisclosed to illustrate the operation of the all-ceramic system. in thisregard the parameters vary depending on the particular glass being made.

The invention is an all-ceramic system which does not contribute toabsorption at 1 micron and which includes a furnace capable of operatingat temperatures up to 1500" C. a crucible to hold the glass, means forstirring which homogenizes the glass and a means for casting the glassinto billets. ln connection with this invention the dimensions of thesystem as disclosed in the examples is not intended to be controlling.

In connection with the improved results which are obtained from theall-ceramic system, over a period of ten months, 75 melts were made insuch a system.

The 75 melts resulted in a total of 868.3 kg of difi'erent types oflaser glass. ln total, 11 different glass types were prepared. Eight ofthese glasses were new, experimental glasses. The other three glasseswere M61838, MG1916 and M01020 of Table V below. 35 melts of the latterglass were made. The composition and information on usable amount ofglass is given in Table V. As determined on 18 melts of MG1838, thereproducibility of the refractive index from melt to melt measured atthe D line is 1.5132 0.0005. The homogeneity of the refractive indexover one full billet was better than one unit in the fifth place asdetermined on one experimental glass melt. This excludes the effect ofvisible striae on the refractive index. The absorption coefficient at1.00 am of 18 melts ofMG1838 glass was (2.5 10.3) cm.

Table V Composition in weight and usable amount of three glasses made inthe all-ceramic melter Glass Components M01838 M01020 M61916 i0 69.2268.52 67.80 Li,0 1.03 1.02 1.01 Na,0 7.43 7.35 7.2! K,O 11.24 11.1311.01 2110 1.55 1.53 1.52 BaO 4.95 4.90 4.85 A1 0, 1.55 1.53 1.52 Sb,0,1.03 1.02 1.01 Nd,0,, 2.00 3.00 4.00 Number of melts made l8 l3 4 Totalamount ofglass in kg 226.8 149.2 51.0 Usable amount ofglass in kg 39.735.4 13.4 Yield in 17.5 23.7 26.]

Accordingly, by providing an all-ceramic system in accordance with theinvention a striae free glass results without metallic inclusion such asplatinum.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes that come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

1 claim:

1. In a system for making glass including a melter positioned within afumace and heating elements for heating said melter, the improvementcomprising a rnuflle interposed between said melter and said heatingelements, said muffle surrounding and extending axially above and belowsaid melter so that homogenization of the temperature distribution insaid melter is accomplished, and said muffle being spaced from saidmelter so that said melter may be separately inserted and withdrawn fromsaid furnace.

2. The system as set forth in claim 1 wherein said mufile has acylindrical shape.

3. A stirrer for homogenizing a glass melt comprising a hollowcylindrical shaft having at least one blade formed thereon, said hollowcylindrical shaft having at least one aperture through a sidewallthereof, said at least one aperture being positioned so as to be locatedbeneath the surface of the glass melt when said stirrer is utilized in amelter, and means for introducing a flow of gas into said hollowcylindrical shaft whereby said at least one blade and gas escaping fromsaid hollow cylindrical shaft through said at least one aperturecooperate in homogenizing the glass melt.

4. The stirrer as set forth in claim 3 wherein said stirrer is formed ofan all-ceramic material.

5. The stirrer as set forth in claim 4 wherein said all-ceramic materialis mullite.

6. The stirrer as set forth in claim 5 wherein said mullite has thefollowing composition in percent by weight:

t sio,

7. The stirrer as set forth in claim 4 wherein said all-ceramic materialconsists essentially of A1 0,, of a purity of at least 96 percent byweight with the balance of the composition being inert impurities.

8. The stirrer as set forth in claim 5 wherein said all-ceramic materialconsists essentially of the following ranges of com position in percentby weight:

Alp, 49 s l ZrO, 3441 sio 1-15 as to be located beneath the surface ofthe glass melt when said stirrer is utilized in said melter, and meansfor introducing a flow of gas into said hollow cylindrical shafi wherebysaid at least one blade and gas escaping from said hollow cylindnicalshaft through said at least one aperture cooperate in homogenizing theglass melt.

10. In a system for making glass including a furnace and a melterwherein the improvement comprises a stirrer for homogenizing glassmelts. said stirrer comprising a hollow cylindrical shaft having a bladeformed thereon and means for introducing a flow of gas into said shaft,wherein said stirrer has at least one aperture positioned to be locatedbeneath the surface of molten glass when the stirrer is utilized in amelter, wherein said stirrer is formed of an all-ceramic material.

11. The stirrer as set forth in claim wherein said a1l ceramic materialis mullite.

12. In a system for making glass including a furnace and a melterwherein the improvement comprises a stirrer for homogenizing glassmelts, said stirrer comprising a hollow cylindrical shaft having a bladeformed thereon and means for introducing a flow of gas into said shaft,including means for rotating said stirrer clockwise andcounter-clockwise.

13. The system as set forth in claim [2 including means for oscillatingsaid stirrer vertically.

14. The system as set forth in claim 13 including means for oscillatingsaid stirrer horizontally.

15. The system as set forth in claim 14 including a cylindrical mufflesurrounding said melter.

16. The system as set forth in claim 14 wherein said melter has ahemispherical bottom having an orifice positioned on the gravitationalcenter of said bottom and an orifice tube integrally formed on saidmelter and surrounding said orifice tube.

17. The system as set forth in claim 16 wherein said hemisphericalbottom is supported on a rotatable hearth.

18. The system as set forth in claim 17 including means for rotatingsaid hearth.

19. The system as set forth in claim 18 wherein said rotatable hearth isstepped to form a stepped bottom on said furnace.

20. The system as set forth in claim 17 wherein said orifice tubeextends into said hearth, said hearth having at least one heatingelement positioned therein for heating said orifice tube.

21. The system as set forth in claim 20 wherein said melter, saidstirrer and said orifice tube are formed of an all-ceramic material.

22. The system as set forth in claim 21 wherein said allceramic materialis mullite.

23. In a system for making glass including a furnace, at melter, astirrer associated with the melter, and a mufile surrounding the melter,wherein the improvement comprises said melter, stirrer, and muffle beingformed essentially completely from an all-ceramic material, saidall-ceramic material being selected from the group consisting ofmullite, alumina of a purity of at least 96 percent by weight, and amaterial which consists essentially of the following ranges ofcomposition in percent by weight:

M20, 49-5 I I0 34-37 SiO: l 1-1 s

1. In a system for making glass including a melter positioned within afurnace and heating elements for heating said melter, the improvementcomprising a muffle interposed between said melter and said heatingelements, said muffle surrounding and extending axially above and belowsaid melter so that homogenization of the temperature distribution insaid melter is accomplished, and said muffle being spaced from saidmelter so that said melter may be separately inserted and withdrawn fromsaid furnace.
 2. The system as set forth in claim 1 wherein said mufflehas a cylindrical shape.
 3. A stirrer for homogenizing a glass meltcomprising a hollow cylindrical shaft having at least one blade formedthereon, said hollow cylindrical shaft having at least one aperturethrough a sidewall thereof, said at least one aperture being positionedso as to be located beneath the surface of the glass melt when saidstirrer is utilized in a melter, and means for introducing a flow of gasinto said hollow cylindrical shaft whereby said at least one blade andgas escaping from said hollow cylindrical shaft through said at leastone aperture cooperate in homogenizing the glass melt.
 4. The stirrer asset forth in claim 3 wherein said stirrer is formed of an all-ceramicmaterial.
 5. The stirrer as set forth in claim 4 wherein saidall-ceramic material is mullite.
 6. The stirrer as set forth in claim 5wherein said mullite has the following composition in percent by weight:Al2O3 63.0 SiO2 37.0
 7. The stirrer as set forth in claim 4 wherein saidall-ceramic material consists essentially of Al2O3 of a purity of atleast 96 percent by weight with the balance of the composition beinginert impurities.
 8. The stirrer as set forth in claim 5 wherein saidall-ceramic material consists essentially of the following ranges ofcomposition in percent by weight: Al2O3 49-51 ZrO2 34-37 SiO2 11-15 withthe balance of the composition being modifying oxides.
 9. In a systemfor making glass including a furnace and a melter wherein theimprovement comprises a stirrer for homogenizing glass melts, saidstirrer comprising a hollow cylindrical shaft having at least one bladeformed thereon, said hollow cylindrical shaft having at least oneaperture through a sidewall thereof, said at least one aperture beingpositioned so as to be located beneath the surface of the glass meltwhen said stirrer is utilized in said melter, and means for introducinga flow of gas into said hollow cylindrical shaft whereby said at leastone blade and gas escaping from said hollow cylindrical shaft throughsaid at least one aperture cooperate in homogenizing the glass melt. 10.In a system for making glass including a furnace and a melter whereinthe improvement comprises a stirrer for homogenizing glass melts, saidstirrer comprising a hollow cylindrical shaft having a blade formedthereon and means for introducing a flow of gas into said shaft, whereinsaid stirrer has at least one aperture positioned to be located beneaththe surface of molten glass when the stirrer is utilized in a melter,wherein said stirrer is formed of an all-ceramic material.
 11. Thestirrer as set forth in claim 10 wherein said all-ceramic material ismullite.
 12. In a system for making glass including a furnace and amelter wherein the improvement comprises a stirrer for homogenizingglass melts, said stirrer comprising a hollow cylindrical shaft having ablade formed thereon and means for introducing a flow of gas into saidshaft, including means for rotating said stirrer clockwise andcounter-clockwise.
 13. The system as set forth in claim 12 includingmeans for oscillating said stirrer vertically.
 14. The system as setforth in claim 13 including means for oscillating said stirrerhorizontally.
 15. The system as set forth in claim 14 including acylindrical muffle surrounding said melter.
 16. The system as set forthin claim 14 wherein said melter has a hemispherical bottom having anorifice positioned on the gravitational center of said bottom and anorifice tube integrally formed on said melter and surrounding saidorifice tube.
 17. The system as set forth in claim 16 wherein saidhemispherical bottom is supported on a rotatable hearth.
 18. The systemas set forth in claim 17 including means for rotating said hearth. 19.The system as set forth in claim 18 wherein said rotatable hearth isstepped to form a stepped bottom on said furnace.
 20. The system as setforth in claim 17 wherein said orifice tube extends into said hearth,said hearth having at least one heating element positioned therein forheating said orifice tube.
 21. The system as set forth in claim 20wherein said melter, said stirrer and said orifice tube are formed of anall-ceramic material.
 22. The system as set forth in claim 21 whereinsaid all-ceramic material is mullite.
 23. In a system for making glassincluding a furnace, a melter, a stirrer associated with the melter, anda muffle surrounding the melter, wherein the improvement comprises saidmelter, stirrer, and muffle being formed essentially completely from anall-ceramic material, said all-ceramic material being selected from thegroup consisting of mullite, alumina of a purity of at least 96 percentby weight, and a material which consists essentially of the followingranges of composition in percent by weight: Al2O3 49-51 ZrO2 34-37 SiO211-15.